In a multi-carrier cellular system, one or more carriers are allocated to each user for data transmission. Orthogonal Frequency Division Multiplexing (Hereinafter, referred to as “OFDM”) is a well-known multi-carrier communication method that is widely used in cellular systems.
The OFDM method is an example of a multi-carrier transmission technique for dividing all transmissible bands into several narrow band sub-carriers, and modulating and transmitting the sub-carriers in parallel. In the OFDM method, a small amount of low-speed data is allocated to each sub-carrier.
Owing to the use of mutually orthogonal sub-carriers, the OFDM method can enhance efficiency of frequency use and overcome a multi-carrier channel using a simple frequency region equalizer having one tap. In recent years, owing to implementation of high speed using Fast Fourier Transform (FFT), the OFDM method is widely used as a transmission method of a high-speed digital communication system. Specifically, in the field of mobile/wireless communications, the OFDM method is used in a Wireless Local Area Network (WLAN), a Wireless Metropolitan Area Network (WMAN), and a cellular mobile communication system.
The multi-carrier cellular system can be configured to allocate a part (one or more) of the sub-carriers to each user and thus provide service to a plurality of users. Here, the carriers allocated to each user can be equally distributed across all bands, or time-dependent frequency hopping can be performed. This is used together with channel coding and interleaving, to obtain an effect of frequency diversity and an effect of averaging interference from adjacent cells in a cellular environment. In the OFDM environment, this is described in detail in J. Chuang and N. Sollenberger, “Beyond 3G: Wideband Wireless Data Access Based on OFDM and Dynamic Packet Assignment”, IEEE Communication Magazine, Volume 38, Issue 7, PP. 78-87, July 2000.
FIGS. 1A and 1B illustrate a conventional frequency hopping method based on a random sequence, wherein FIG. 1A illustrates carrier allocation in a cell “A” and FIG. 1B illustrates carrier allocation in a cell “B”.
In FIGS. 1A and 1B, the vertical direction of a lattice corresponds to frequency and reference numeral 11 denotes one sub-carrier. Further, the horizontal direction of the lattice represents time and reference numeral 10 denotes a symbol period. Reference numeral 12 denotes a unit of channel coding. That is, one channel is comprised of nine symbols. Further, the carrier allocated to each user is allocated on the basis of the random sequence.
FIG. 1A shows an example of a three-channel construction format within the cell “A”. FIG. 1B shows an example of a one-channel construction format within the cell “B”. It is assumed that the cell “A” and the cell “B” are adjacent or in very close proximity to each other. A channel construction format (frequency allocation or hopping pattern) should be different between the adjacent or closely positioned cells in order to average interference from the adjacent cells. If the two closely positioned cells use the same hopping pattern, lasting and heavy interference is caused between the same channels. In the examples of FIGS. 1A and 1B, for a user 0 of the cell “A” and a user 0 of the cell “B”, the interference is caused only in 3 out of the 9 symbol periods which constitute one channel coding period. In other words, the interference is not concentrated only in one specific channel, but rather an interference averaging effect occurs such that there is relatively equal interference in other channels as well. Consequently, the cells within a mobile communication network based on a frequency hopping OFDM each have an inherent hopping pattern, and closely positioned cells have different hopping patterns from one another, thereby averaging the influence of interference from adjacent cells. For the frequency hopping pattern (channel construction format), a conventional method uses a pattern formed using a pseudo random sequence.
Assuming that the randomly generated frequency hopping pattern is formed as in FIGS. 1A and 1B, a level of interference affecting the user 0 of the cell “B” and each user of the cell “A” will be described. As shown in the drawings, the user 0 of the cell “A” experiences interference only for 3 symbols. Whereas, the user 1 of the cell “A” experiences interference only during 2 symbols, and thus experiences less interference. However, the user 2 experiences interference during 4 symbols, and therefore is subjected to heavy interference. The above frequency collision frequently occurring between specific channels causes heavy interference, thereby causing a high Bit Error Rate (BER) and deteriorating the performance of a system. In a case where the total number of symbols is 3*9=27 and the number of channels (the number of simultaneous users) is 3, as in the examples of FIGS. 1A and 1B, the best hopping pattern in terms of interference averaging is where the interference occurs only during the 3 symbols between all channels. Thus, the frequency allocation or hopping pattern formed by the pseudo random sequence has a drawback in that due to irregularity of a level of interference between the channels of two adjacent cells, complete interference averaging cannot be performed.
Further, in addition to the aforementioned adjacent cell interference averaging, the carrier allocation and hopping pattern of the multi-carrier cellular system should also average interference coming from other users within the same cell. In general, in the multi-carrier system, the carriers are orthogonal resources which do not interfere with one another. Therefore, if carriers different from one another are allocated and data is transmitted to the users, no interference occurs. However, even when different users transmit data in an upward link using different carriers, when adjacent carriers are used and the users movement speeds are different, the Doppler effect can render the adjacent carriers similar enough to interfere with one another. Therefore, it is necessary to average allocation of adjacent carriers between users.
Referring again to the example of FIG. 1, there are two cases in which the user 1 of the cell “A” uses the underlying sub-carrier of the user 0, but there are four cases in which the user 0 uses the underlying sub-carrier of the user 2. If adjacent carriers are used between specific channels many times, adjacent channel interference is heavy, resulting in a high BER and deteriorating the performance of the system. Therefore, it is necessary to average the number of times adjacent channels are allocated.